1. Field of the Invention
The present invention relates to plating palladium on copper and, more particularly, to a method of electroless plating of palladium phosphorus alloy directly onto a copper surface, and the product resulting from the plating method.
2. Description of Related Art
Metal plating is used in many industries for various reasons. For example, metal plating is used in the electronics industry to increase the ability to solder to a base metal, to increase resistance to corrosion, to alter conductivity, and for radiation shielding, among others. Various metals are used for plating and each metal has its own attributes. For example, nickel is relatively easy to plate onto a base metal and is relatively inexpensive; however, it has a relatively low oxidation resistance and malleability (meaning it may have a tendency to crack under stress). On the other hand, gold is relatively oxidation resistant yet is relatively expensive and has a tendency to tarnish from copper diffusion into the gold forming an oxidized layer when plated directly on copper. Palladium is highly corrosion resistant, is relatively easy to solder to, and enables gold wire bonding when used in conjunction with gold; however, it is difficult to plate the phosphorus alloy directly on copper surfaces.
In some situations, multiple materials are plated on a substrate in order to achieve a desired set of qualities. For example, a copper surface may receive a nickel plating with a gold plating on the nickel plating. This combination allows the final coating to have the desirable properties of gold, and the nickel plating blocks the copper diffusion and the resulting tarnish of the gold. However, the combination of gold plating and nickel plating is not typically suitable for gold wire bonding with the exception of gold thicknesses well above 0.1 um, which can undesirably increase the plating costs. Palladium can be plated between the nickel and the gold layers, increasing the suitability of the plating for wire bonding. This process can be relatively expensive and time consuming as it utilizes three distinct materials and plating processes. There are additional drawbacks. For example, nickel plating is traditionally both thick and rigid, making it unsuitable for flexible printed circuit boards (PCBs) as well as when distances between components to be plated are relatively small. Money, time, and real estate costs could be reduced and usability increased by eliminating the nickel plating and plating the palladium phosphorous directly on the copper. However, a suitable method for plating palladium phosphorous directly onto copper is not known in the art.
Electrolytic plating is one available plating method but it is typically unsuitable for electronic components. Plating by electrolytic process requires the base metal (i.e., the copper and a second metal part) to each be submerged into a solution of water soluble metal salts. A current is applied to the base metal and the second metal part, making the base metal a cathode and the second metal part an anode. The electrical current reduces the ionic metal in the solution, forming a solid metal coating on the cathode (the base metal). However, electrolytic plating has a significant drawback—any surface to be coated must be made cathodic. Thus, electrolytic plating is unsuitable for coating any component having multiple insulated surfaces to be plated.
Immersion plating is another plating method used in the art, yet is typically unsuitable for plating palladium directly on copper. Immersion plating includes submerging a base metal into a solution having electrolytes and metal salts. When the base metal is submerged, the electrolytes corrode the surface of the base metal, making the surface electrically cathodic relative to the dissolved metal. The metal salts in the solution are then reduced, forming a metal plate on the base metal. A first limitation with immersion plating is that the plating is limited to a relatively thin layer because as soon as the surface of the base metal is coated, the reaction can no longer occur. Another limitation with immersion plating, especially of palladium onto copper, is that the resulting palladium coating is granular and porous, making it unsuitable for soldering and wire-bonding.
Some attempts have been made for electroless plating (autocatalytic plating) of palladium phosphorus onto copper; however, no consistent or commercially suitable results have yet been obtained. Electroless plating is performed by creating a bath including a metal salt and a reducing agent. When the reducing agent is exposed to a catalyst (typically the surface of the substrate to be plated or a film thereon), the reducing agent donates electrons, causing the metal salt to precipitate on to the surface of the substrate. Accordingly, for electroless plating to work, the surface of the substrate must be catalytic. Nickel, palladium, and cobalt are examples of known catalysts for electroless palladium phosphorus plating. Copper, however, is not a catalyst for palladium plating, and in fact is a catalytic poison, meaning that it effectively prevents the electroless plating reaction.
There is an unfulfilled need for a component that has palladium phosphorus plated directly on a copper surface, a method for suitably plating palladium onto copper, and a bath composition for performing the plating. The inventor has found a solution to this unfulfilled need.